Lithium tantalate based X-ray intensifying screen

ABSTRACT

A novel X-ray intensifying screen is detailed comprising Li3Ta1-xNbxO4 wherein x is 0.0 to 1.0. Further detailed is a radiographic recording element employing the same screen. This novel X-ray intensifying screen has improved speed when compared to X-ray intensifying screens comprising CaWO4.

FIELD OF INVENTION

This invention relates to radiographic intensifying screens forconverting image-wise modulated X-radiation into ultraviolet and/or blueradiation. More specifically this invention is related to lithiumtantalate phosphors for use in a radiographic intensifying screen.

BACKGROUND OF THE INVENTION

X-ray excited phosphors are well known for their use in medicalradiography. Medical X-ray films are typically sensitive to UV, blue orgreen light and therefore the X-radiation containing the diagnosticinformation must be converted to lower energy to optimally expose thefilm. This conversion is done by an intensifying screen comprising anappropriate phosphor as the principal active ingredient.

Phosphors which are suitable for converting X-ray energy to theappropriate lower energy are well known. The specific desirableproperties include: high conversion efficiency, low granularity and alow quantity of afterglow emission. Particularly useful are phosphorsfor use in x-ray intensifying screens which have an emission in the nearUV and blue. The most widely known phosphor for this purpose is CaWO₄which has become the industry standard. There is an ongoing desire inthe art to provide UV or blue emitting medical x-ray intensifyingscreens which have a higher speed than intensifying screens comprisingCaWO₄.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an x-rayintensifying screen for use in radiographic elements. Other advantages,as will become apparent from the teachings herein are provided in anx-ray intensifying screen comprising a support at least one active layercoated on said support; wherein said active layer comprises a binder anda phosphor corresponding to the formula

    Li.sub.3 Ta.sub.1-x Nb.sub.x O.sub.4

wherein x is 0 to 1.0.

A particularly preferred embodiment is obtained when the x-rayintensifying screen of the present invention is used in combination witha suitable film as provided in a radiographic recording elementcomprising at least one x-ray intensifying screen in operativeassociation with a photosensitive film element wherein said x-rayintensifying screen comprises; a support; at least one active layercoated on said support and said active layer comprises a phosphorcorresponding to the formula:

    Li.sub.3 Ta.sub.1-x Nb.sub.x O.sub.4

wherein x is 0.0 to 1.0.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating the relative speed of Li₃ Ta_(1-x) Nb_(x)O₄ as x increases from 0.0 to 0.10.

FIG. 2 is a graph illustrating the relative speed of Li₃ Ta_(1-x) Nb_(x)O₄ as x increases from 0.0 to 0.60.

FIG. 3 is a graph illustrating the maximum emission of an X-ray excitedLi₃ Ta_(1-x) Nb_(x) O₄ wherein x is 0.50 to 1.0. The discontinuityrepresents the structural transition from monoclinic morphology to cubicmorphology.

FIG. 4 is a graph illustrating the maximum emission of an X-ray excitedmonoclinic Li₃ Ta_(1-x) Nb_(x) O₄ wherein x is 0.0 to 0.6.

DETAILED DESCRIPTION OF THE INVENTION

The screen of the present invention comprises a phosphor whichcorresponds to the formula:

    Li.sub.3 Ta.sub.1-x Nb.sub.x O4

wherein x is 0 to 1.0. Preferred is a phosphor where x is 0 to 0.7, morepreferred is a phosphor where x is 0 to 0. 1 and even more preferred isa phosphor where x is 0 to 0.07. The most preferred phosphor is obtainedwhen x is between 0.002 and 0.010. When x is outside of the range of0.002 to 0.010 a drop in intensity is observed as indicated in FIG. 1wherein "Sp" is speed relative to High Plus arbitrarily defined as 1.0.

Preparation of the phosphor of the present invention is well documentedin the art and yet the use of this phosphor in an X-ray intensifyingscreen has not been previously contemplated. The preparation of thephosphor is known to proceed according to following reaction:

    (1-x)Ta.sub.2 O.sub.5 +xNb.sub.2 O.sub.5 +3Li.sub.2 CO.sub.3 →2Li.sub.3 Ta.sub.1-x Nb.sub.x O.sub.4 +3CO.sub.2

The preparative procedure typically involves intimately mixing theproper ratios of the phosphor precursors, such as Li₂ CO₃, Ta₂ O₅ andNb₂ O₅ in a suitable flux such as Li₂ SO₄ or LiCl. This mixture isheated in an alumina crucible at 1000° C. The flux is removed bydissolving in water, the phosphor is recovered and a preselectedparticle size distribution is used for the preparation of an X-rayintensifying screen.

After firing in the conventional manner, flux and soluble reactionproducts are removed by water leaching or some other suitable method.The resulting Li₃ Ta_(1-x) Nb_(x) O4 (x=0 to approximately 0.7)crystallizes in a monoclinic C2/c space group or in a P2/n space groupas detailed in Journal of Solid State Chemistry, 48, 420-430 (1983). Thecrystallographic forms are also detailed in J. of Solid State Chemistry,53, 277-278 (1984). At stoichiometric levels of niobium aboveapproximately 0.7 the structure is cubic with a space group symmetry of123 and the properties of the phosphor become less desirable for use inan x-ray intensifying screen.

The stoichiometric level of niobium directly impacts the emissionmaximum of the phosphor. From a stoichiometric level of 0.0 to 0.6 theemission maximum, under x-ray excitation, increases from approximately410 nm to approximately 435 nm as illustrated in FIG. 4. At niobiumlevels above approximately 0.7 the cubic structure is observed and theemission, under x-ray excitation, increases with niobium level. Theemission is lower for the cubic structure than for the monoclinicstructure.

There are a host of fluxes which can be used to prepare the phosphordescribed above. These include the alkali metal salts, e.g., lithium,sodium, potassium, etc., of sulfates, phosphates, carbonates,metaborates, etc. The halides are also known to be beneficial when addedto the flux in combination with sodium, lithium, potassium, calcium,strontium, barium and the like. Particularly suitable fluxes aredescribed, for example, in Hedden and Zegarski, U.S. Pat. No. 4,250,365.The amount of flux is not limiting. Typically a flux is present in thereaction mixture in about 30% to about 60% by weight, preferably about45% to about 55% by weight, based on the total weight of oxide (phosphorstarting material). The optional flux and phosphor precursors areintimately mixed by any of a number of means including ball milling,shaking, or by the use of a flowing gaseous or liquid medium such asair, nitrogen, water, fluorochlorinated hydrocarbons or other inertfluids as detailed in U.S. Pat. No. 5,154,360. The mixture of phosphorstarting materials, and optional flux can be fired, e.g., for at leastabout three hours, at elevated temperatures, e.g., from 750° C. to 1500°C., before washing to remove a majority of the flux and recovering thephosphor.

After firing, pulverizing and washing, the phosphor is mixed with asuitable binder in the presence of a suitable solvent and coated on asupport. All of these steps are described in the U.S. Pat. No. 4,225,653and all are well-known in the prior art. A protective topcoat may alsobe applied over this phosphor coating, in fact it is preferred.

The phosphor may be dispersed in any of the commonly known binders,e.g., polyvinyl butyral or the acrylates or methacrylates, using asolvent, e.g., an alcohol, chlorinated hydrocarbon, ketone, butylacetate, etc. Small amounts of fumed silica may be present in thephosphor to enhance handling and make the phosphor easier to pour. Afterdispersing in the binder, the phosphor is then cast on a conventionalsupport, e.g., cardboard, polyester film, thin metal sheets, etc. Abrightener may also be present within the phosphor and variousreflective materials may be present as an underlayer or within thesupport itself to enhance the generation of light when the phosphor isstruck by X-radiation. TiO₂ dispersed in a binder and cast on thesupport is conventional as well as the use of small particles of rutileTiO₂ directly in a film support. The phosphor is typically applied tothe support in an amount ranging from 15 to 110 mg/cm². All of theseprocedures are well-known in the art. Over the phosphor layer which hasbeen cast on the support, a conventional protective topcoat may beapplied and, in fact, is preferred. These topcoats are also well-knownin the prior art and serve to protect the rather expensive phosphorlayer from stains and defects during the handling thereof. Aparticularly suitable topcoat is achieved when a styrene/acrylonitrilecopolymer is coated supra to the phosphor layer and dried. Conventionalsupports, binders, mixing and coating processes for the manufacture oftypical X-ray intensifying screens are, for example, described in PattenU.S. Pat. No. 4,387,141, the pertinent disclosure of which isincorporated herein by reference thereto.

The screens of the present invention may be used in pairs in cooperationwith double-side coated medical X-ray silver halide photographic filmelements, although it is contemplated that single-side coated silverhalide photographic film elements may be used for some applications. Ifa pair of screens is used the coating weights of each screen may bedifferent, if required. Thus, an asymmetric pair of screens can be usedto get the best results. Medical X-ray evaluations represent thepredominant commercial use for X-ray intensifying screens of thisinvention. A dimensionally stable, polyethylene terephthalate filmsupport into which small amounts of rutile or anatase titanium dioxidehave been incorporated is the preferred support for the x-rayintensifying screen of this invention.

In operation, the intensifying screen absorbs X-rays that impingethereon and emits energy having a wavelength that is readily captured bythe photographic silver halide X-ray film associated therewith. Thesescreens are generally prepared according to the methods of Brixner, U.S.Pat. No. 4,225,653.

In the radiological process, it is conventional to employ aphotosensitive silver halide film element with the above described X-rayintensifying screens. In the practice of this invention, the silverhalide element will be comprised of silver halide grains. These elementsare also well-known in the prior art and the preparation of grains arealso known and taught therein. The grains are generally made into anemulsion using a binder such as gelatin, and are sensitized with goldand sulfur, for example. Other adjuvants such as antifoggants, wettingand coating aides, sensitizing dyes, hardeners etc. may also be presentif necessary. The emulsion may be double-side coated on the support anda thin, hardened gelatin overcoat is usually applied over each of theemulsion layers to provide protection thereto. If required, a smallamount of a sensitizing dye might advantageously be added. Additionally,it is also conventional to add a sensitizing dye to tabular emulsions inorder to increase their ability to respond to light.

The silver halide emulsion may employ any of the conventional halidesbut preferred are pure silver bromide or silver bromide with smallamounts of iodide incorporated therein (e.g. 98% Br and 2% I by weightfor example). Any grain morphology is suitable for demonstration ofthese teachings including, but not limited to, grains which are formedby splash techniques and those formed by techniques involving spraytechniques (i.e. single and double jet procedures). Tabular grains aremost preferred.

Tabular grain silver halide products are well-known in the prior artwith exemplary methods of manufacture described by Maskasky in U.S. Pat.No. 4,400,463; Wey, U.S. Pat. No. 4,399,205; Dickerson, U.S. Pat. No.4,414,304; Wilgus et al., U.S. Pat. No. 4,434,226; Kofron et al., U.S.Pat. No. 4,439,520; Nottorf, U.S. Pat. No. 4,722,886; and Ellis, U.S.Pat. No. 4,801,522.

After the grains are made, it is usually preferable to disperse thegrains with a binder (e.g. gelatin or other well-known binders such aspolyvinyl alcohol, phthalated gelatins, etc.). In place of gelatin othernatural or synthetic water-permeable organic colloid binding agents canbe used as a total or partial replacement thereof. Such agents includewater permeable or water-soluble polyvinyl alcohol and its derivatives,e.g., partially hydrolyzed polyvinyl acetates, polyvinyl ethers, andacetals containing a large number of extralinear -CH₂ HOH-- groups;hydrolyzed interpolymers of vinyl acetate and unsaturated additionpolymerizable compounds such as maleic anhydride, acrylic andmethacrylic acid ethyl esters, and styrene. Suitable colloids of thelast mentioned type are disclosed in U.S. Pat. Nos. 2,276,322, 2,276,323and 2,347,811. The useful polyvinyl acetals include polyvinylacetalaldehyde acetal, polyvinyl butyraldehyde acetal and polyvinylsodium o-sulfobenzaldehyde acetal. Other useful colloid binding agentsinclude the poly-N-vinyllactams of Bolton U.S. Pat. No. 2,495,918, thehydrophilic copolymers of N-acrylamido alkyl betaines described inShacklett U.S. Pat. No. 2,833,650 and hydrophilic cellulose ethers andesters. Phthalated gelatins may also be used as well as binder adjuvantsuseful for increasing covering power such as dextran or the modified,hydrolyzed gelatins of Rakoczy, U.S. Pat. No. 3,778,278.

It is most preferable to chemically sensitize the grain with salts thatare well known in the art. The most common sensitizers are salts of goldor sulfur. Sulfur sensitizers include those which contain labile sulfur,e.g. allyl isothiocyanate, allyl diethyl thiourea, phenyl isothiocyanateand sodium thiosulfate for example. Other non-optical sensitizers suchas amines as taught by Staud et al., U.S. Pat. No. 1,925,508 andChambers et al., U.S. Pat. No. 3,026,203, and metal salts as taught byBaldsiefen, U.S. Pat. No. 2,540,086 may also be used.

The emulsions can contain antifoggants, e.g. 6-nitrobenzimidazole,benzotriazole, triazaindenes, etc., as well as the usual hardeners,i.e., chrome alum, formaldehyde, dimethylol urea, mucochloric acid, andothers as recited in Research Disclosure, No. 308, December 1989, Item30819. Other emulsion adjuvants that may be added comprise mattingagents, plasticizers, toners, optical brightening agents, surfactants,image color modifiers, non-halation dyes, and covering power adjuvantsamong others.

Thermographic imaging emulsions comprising silver salts of carboxylicacids may also be used with the screens of the present invention.Exemplary examples are provided in U.S. Pat. No. 5,028,518.

The film support for the emulsion layers used in the process may be anysuitable transparent plastic. For example, the cellulosic supports, e.g.cellulose acetate, cellulose triacetate, cellulose mixed esters, etc.may be used. Polymerized vinyl compounds, e.g., copolymerized vinylacetate and vinyl chloride, polystyrene, and polymerized acrylates mayalso be mentioned. Preferred films include those formed from thepolyesterification product of a dicarboxylic acid and a dihydric alcoholmade according to the teachings of Alles, U.S. Pat. No. 2,779,684 andthe patents referred to in the specification thereof. Other suitablesupports are the polyethylene terephthalate/isophthalates of BritishPatent 766,290 and Canadian Patent 562,672 and those obtainable bycondensing terephthalic acid and dimethyl terephthalate with propyleneglycol, diethylene glycol, tetramethylene glycol or cyclohexane1,4-dimethanol (hexahydro-p-xylene alcohol). The films of Bauer et al.,U.S. Pat. No. 3,052,543 may also be used. The above polyester films areparticularly suitable because of their dimensional stability.

When polyethylene terephthalate is manufactured for use as aphotographic support, the polymer is cast as a film, the mixed polymersubbing composition of Rawlins, U.S. Pat. No. 3,567,452 is applied andthe structure is then biaxially stretched, followed by application of agelatin subbing layer. Alternatively, antistatic layers can beincorporated as illustrated, for example, by Miller, U.S. Pat. Nos. 4,916,011 and 4,701,403, Cho, U.S. Pat. Nos. 4,891,308 and 4,585,730 andSchadt, U.S. Pat. No. 4,225,665. Upon completion of stretching andapplication of subbing composition, it is necessary to remove strain andtension in the base by a heat treatment comparable to the annealing ofglass.

The emulsions may be coated on the supports mentioned above as a singlelayer or multi-layer element. For medical X-ray applications, forexample, layers may be coated on both sides of the support whichconventionally contains a dye to impart a blue tint thereto. Contiguousto the emulsion layers it is conventional, and preferable, to apply athin stratum of hardened gelatin supra to said emulsion to provideprotection thereto.

Medical X-ray film processing is well documented in the art asexemplified in Wuelfing, U.S. Pat. No. 4,741,991. A medical X-ray filmto be processed is typically developed to convert latent image centerswithin the silver halide grain into elemental silver. Unreacted silverhalide is then removed by dissolving in a suitable fixer and the film iswashed and dried to provide a suitable image.

The speed in a photographic system is broadly defined as the exposurerequired to obtain a predetermined density under standard processingconditions. For a medical X-ray system the specific procedure isdetailed in ANSI Standard, PH2.9, 1964. It is widely accepted in the artto report a relative speed wherein the speed is determined relative toPAR which is arbitrarily assigned a speed value of 100. PAR speed isdetermined with a standard CaWO₄ screen with a 84 μm phosphor thicknessin combination with a Du Pont Cronex® 4 film. High Plus screens and Parscreens utilize a CaWO₄ phosphor and are available from E. I. DuPont deNemours, and Co. Relative speed for a medical X-ray system is determinedat a density of 1.0 above base plus fog density by exposure modulationtechniques as exemplified in SPSE HANDBOOK OF PHOTOGRAPHIC SCIENCE ANDENGINEERING, Woodlief, Ed.; John Wiley and Sons, New York, 1973, pp.798-800.

EXAMPLES Phosphor/Screen Preparation

7.9384 g. of Ta₂ O₅, 0.0240 g. of Nb₂ O₅, 4.0000 g. of Li₂ CO₃, and 5.98g. of Li₂ SO₄ were ground together in a mortar and transferred to acovered alumina crucible. The sample was heated in air to 1000° C. overa period of 1.75 hours and held at 1000° C. for 10 hours followed byfurnace cooling. The mixture was then washed with water to remove theflux and the resulting Li₃ Ta₀.995 Nb₀.005 O₄ was dried and collected.The X-ray powder diffraction pattern of this compound shows only thelines for the monoclinic (psuedo-tetragonal) structure. The phosphor wasincorporated into a screen using a polyvinyl butyral binder inaccordance with the teachings of Brixner U.S. Pat. No. 4,225,653. Thespeed was measured using Cronex®10T medical X-ray film (available fromE.I. DuPont de Nemours, and Co. of Wilmington, Del.) exposed at 70 kVpwith a tungsten tube. The results are contained in Table 1.

                  TABLE 1                                                         ______________________________________                                        Relative Speed of Inventive X-ray Intensifying Screen                         Phosphor    Coating Weight                                                                            Speed*                                                ______________________________________                                        Par         --          100      Comparative                                  High Plus   --          216      Comparative                                  Li.sub.3 Ta.sub.0.995 Nb.sub.0.005 O.sub.4                                                48.4 mg/cm.sup.2                                                                          271      Inventive                                    Li.sub.3 Ta.sub.0.995 Nb.sub.0.005 O.sub.4                                                36.9 mg/cm.sup.2                                                                          234      Inventive                                    Li.sub.3 Ta.sub.0.995 Nb.sub.0.005 O.sub.4                                                28.2 mg/cm.sup.2                                                                          192      Inventive                                    ______________________________________                                         *Speed is relative to Par which is arbitrarily defined as a speed of 100.

Emission changes as a function of niobium level

A series of samples were prepared as detailed above with the level ofniobium corresponding to x values of 0.0, 0.002, 0.005, 0.01, 0.02,0.10, 0.50, 0.60, 0.80, 1.00. The emission maximum was obtained with aHamamatsu R928 photomultiplier tube. FIG. 2 contains a plot whereinx=0.0 to 0.6. In FIG. 2 "Sp" is speed relative to High Plus which isarbitrarily defined as 1.0. FIG. 3 contains a graph of speed relative toHigh Plus arbitrarily defined as 1.0 (Sp) wherein x=0.5 to 1.0. In FIG.3 the transition period between monoclinic and cubic structure isindicated by the void between 0.6 and 0.8. This region has not beenanalyzed to determine the exact transition composition. Thecorresponding emission maximum (EM) is shown in FIG. 4 for x=0.0 to 0.6.

I claim:
 1. An x-ray intensifying screen comprisinga support at leastone active layer coated on said support; wherein said active layercomprisesa binder; a phosphor corresponding to the formula

    Li.sub.3 Ta.sub.1-x Nb.sub.x O.sub.4

wherein x is 0 to 1.0.
 2. The x-ray intensifying screen recited in claim1 wherein x is 0.0 to 0.7.
 3. The x-ray intensifying screen recited inclaim 2 wherein x is 0.0 to 0.1.
 4. The x-ray intensifying screenrecited in claim 3 wherein x is 0.0 to 0.07.
 5. The x-ray intensifyingscreen recited in claim 4 wherein x is 0.002 to 0.010.
 6. A radiographicrecording element comprising at least one x-ray intensifying screen inoperative association with a photosensitive film element wherein saidx-ray intensifying screen comprises;a support; at least one active layercoated on said support wherein said active layer comprises a phosphorcorresponding to the formula:

    Li.sub.3 Ta.sub.1-x Nb.sub.x O.sub.4

wherein x is 0.0 to 1.0.
 7. The radiographic recording element recitedin claim 6 wherein x is 0.0 to 0.7.
 8. The radiographic recordingelement recited in claim 7 wherein x is 0.0 to 0.1.
 9. The radiographicrecording element recited in claim 8 wherein x is 0.0 to 0.07.
 10. Theradiographic recording element recited in claim 9 wherein x is 0.002 to0.010.
 11. The radiographic recording element recited in claim 6 whereinsaid photosensitive recording element comprises silver halide.